1. Field of the Invention
This invention relates generally to a sonic energy transmitter for use in logging-while-drilling (LWD), and particularly to a sonic energy transmitter that is constructed and mounted on a drill collar in a manner such that coupling of energy into the drill collar is substantially suppressed.
2. Description of the Related Art
Sonic logging-while-drilling is highly useful in obtaining information concerning the properties of an earth formation that has been penetrated by a drill bit. Sonic LWD permits the travel time of a compressional wave in the formation to be determined. This travel time is important and useful for seismic interpretation and processing because the travel time of a compressional wave in fluid saturated rock is strongly influenced by the porosity of the rock and the type of fluid it contains. Good porosity is necessary for the formation to contain oil or gas in commercial quantities.
In the typical wireline sonic logging technique a transmitter produces acoustic waves which travel through the rocks around the borehole toward several spaced receivers which detect the arrival of the waves in a manner such that the travel time of compressional waves in the rocks can be determined. Sonic LWD measurements are highly desirable compared to wireline measurements because they are made before borehole conditions deteriorate due to alteration of the formation by the drilling mud, loss of the hole under difficult formation conditions, and roughening of the borehole wall due to repeated bit trips. Sonic LWD also eliminates the delays associated with wireline logging, during which drilling operations must be suspended.
A sonic energy transmitter useful in LWD should meet a number of requirements. It must be rugged enough to withstand the drilling environment which includes vibration and shock loads and must output pressure waves strongly and efficiently to the surrounding formation. However, the transmitter should not excite vibrations that could travel along the drill collar itself and cause interference with vibrations returning through the rocks, since these so called collar arrivals seriously degrade the measurement. Therefore, the transmitter should be mounted in a manner that reduces to a minimum the coupling of vibration energy to the collar.
Collar arrivals can also be attenuated by a section of periodic grooves between the transmitter and the receivers. This means of attenuation is most effective for symmetric vibrations, and asymmetric (flexural) vibrations coupled by the transmitter to the collar should therefore be minimized.
An object of the present invention is to provide a new and improved sonic energy transmitter for LWD applications which incorporates the desirable features noted above.
Another object of the present invention is to provide a sonic transmitter of the type described which is constructed, arranged and mounted in a manner such that coupling of energy into the drill collar is minimized.
Still another object of the present invention is to provide a transmitter of the type described which is arranged to decrease the amount of asymmetric energy generated.